1. Field of the Invention
This invention generally relates to the art of ultra-low frequency subterranean, electromagnetic telemetry, and more particularly, to a method and apparatus for recovering telemetry data packets in the presence of strong interfering ambient noise.
2. Description of the Related Art
Ultra-low frequency (ULF) electromagnetic (EM) waves are the preferred transmission mechanism for wireless subterranean telemetry applications due to the ULF wave's ability to propagate long distances through the earth's strata. In a typical subterranean telemetry application, the desired telemetry information is digitally encoded into data packets and sent as modulated “bursts” of ULF carrier waves. Transmission of the carrier waves is physically facilitated by injecting a modulated current into the earth media using a power amplifier to create a time-varying voltage potential between two transmit electrodes coupled to the earth media. The electrodes are spaced such that the induced current traverses a section of the earth media creating associated electric and magnetic field energy which radiates as time-varying wave fronts through the earth media.
Ultra-low frequency EM waves have the potential for traveling many thousands of feet through an earth media. The actual wave propagation distance is dependent upon several variables, the predominate variables being related to the geophysical characteristics of the earth strata imposed between the transmitter and a remote receiver. As with any communication system, the lossy nature of the transmission media will result in a degradation of power of the EM waves as they traverse the media. This loss of power is proportional to the distance traversed within the media; thus, the overall received signal strength can be greatly attenuated when it reaches a remotely located receive antenna.
The traveling EM waves are recovered at the receive end of the transmission link using a pair of receive electrodes which are spaced within the earth media so as to receive the incident voltage potential of the arriving EM wave fronts. Most commercial EM receivers employ some type of highly sensitive front-end amplifier, connected to the receive electrodes, to boost the strength of the received signal. However, even with the application of front-end amplification, the attenuation of the EM wave energy during traversal through an earth media can be so great that serious degradation of the signal-to-noise (SNR) ratio incident at the telemetry receiver will result. This SNR degradation is further compounded when the receive elements are located within an electrically noisy ambient environment, where both the arriving EM signal and the attendant surface noise receive equal amplification by the receiver front-end.
A common application area for ULF EM telemetry is borehole to surface communications, with the primary market relating to energy exploration and mining operations. Noise incident at the surface receiver is a major problem for borehole telemetry applications due to the harsh nature of the operational environment. This problem is compounded due to the fact that the wave-mechanics associated with downhole to surface EM wave propagation requires that the receive electrodes of the surface receiver be located proximal to the borehole and thus near the radiant noise sources located on or near the rig. A working rig creates a high energy, constantly changing ambient electrical noise environment due to the proximity of electric motors, switch and relay arcing, contact of dissimilar metals, the presence of high-voltage/current power conductors, etc. This ambient noise, in conjunction with the arriving, attenuated EM wave fronts, can cause severe degradation of the surface receiver SNR, making borehole telemetry operations unreliable, or in extreme cases, nonfunctional. The elevated noise environment presented by the rig makes the application of some form of receiver noise rejection mandatory for a practical realization of any type of commercially viable borehole telemetry system.
Borehole EM telemetry systems have been experimentally proposed and commercially produced for a number of years, with some of this work resulting in patents, the earliest found to date being in 1935 by J. H. Clark. Initial work in EM telemetry continued for a number of years, with several publications and patents coming from Daniel Silverman during the 1940's.
Most currently available ULF EM borehole telemetry systems utilize some form of noise rejection at the surface receiver in order to boost the receiver SNR and thus improve operational reliability and extend the telemetry depth capability. Early systems utilized hardware based electronic band-pass filters to discriminate against noise which lay outside the carrier frequency information bandwidth. Although adequate for discriminating against out-of-band noise (i.e. noise which resides at least one octave above or below the carrier frequency), hardware based filters provide little to no rejection for noise which is coincident at or near the carrier frequency.
Recent advances in microcomputer technology have allowed ULF EM receiver designers to employ increasingly sophisticated signal processing techniques to reject noise which is resident both in and out of the carrier information bandwidth. A common technique utilizes supplemental receive sensors to monitor the ambient noise environment. Noise data acquired by these noise sensors is processed by the receiver and used to alter the primary receive signal such that the noise within the primary signal is attenuated. There are several instances cited within the prior art where different variants of this type of multi-channel receiver topology is used to facilitate noise rejection for ULF bandwidth EM applications.
U.S. Pat. No. 4,750,156 describes a noise suppression method for application to seismic monitoring. Specifically, a separate noise receive detector is used to monitor the contaminating noise signal. The detected noise is processed by the receiver and used to generate a reference noise signal which is subsequently used to alter the original seismic signal such that the contaminating noise is minimized within the original signal.
U.S. Pat. Nos. 4,980,682, 5,189,414, and 6,657,597 describe various methods of reducing noise in borehole telemetry systems using multiple signal and noise sensors. Each of these methods utilizes a technique of simultaneously monitoring signal and noise at the receiver using multiple receive sensors. In each method cited, the information received from the noise sensors is utilized (using various signal processing techniques) to actively attenuate the noise from the original received signal.
U.S. Pat. No. 5,157,605 describes a method of combining multiple electromagnetic signals to facilitate improved reception during induction well logging. The technique described combines multiple receive signals using a weighted averaging scheme, in conjunction with multiple transmit frequencies, to improve the depth of investigation and vertical resolution of the induction logging measurement.
The examples cited from the prior art achieve noise rejection through direct alteration of the raw carrier signal waveform via spectral or temporal manipulation of incoming noise-waveform information gathered from supplemental signal sources. Signal processing techniques which require direct real-time combinatorial manipulation of the raw incoming data streams are computationally intensive. The computational burden is greatly increased when multiple sensor channels must be monitored and processed in real-time. An additional computational burden is placed on this type of multi-channel receiver in that the receiver must somehow decide which receive channels to process as “noise” channels and which to process as “signal” channels.
Finally, the cited prior art utilizes only the most basic information regarding the specific temporal or spectral noise content of the signal. No inference is made (or utilized) regarding the higher level information content of received telemetry signal metrics such as the specifics of the modulation scheme or the modulation protocol structure. The supplemental knowledge of such metrics can provide valuable information regarding the reconstruction of a noise corrupted message data packet.
Identification of Objects of the Invention. It is generally an object of the invention described herein to provide improved rejection of noise during the reception of ultra-low frequency EM telemetry data packets.
It is a specific object of the invention to provide an improved low-frequency EM telemetry receiver apparatus which fuses multiple receive-input sources to synthesize a decodable telemetry data packet.
It is a further object of the invention to provide a telemetry data packet synthesis method whereby a set of predetermined signal metrics are used to establish a “confidence” rating for each modulated frame of telemetry data being simultaneously received by multiple receive-input sources, whereby a single decodable telemetry data packet can be assembled using selected frames from all available receive-input sources.